The present invention relates to an apparatus for detecting the A/F (air/fuel) ratio of an exhaust gas from a burner device such as an internal combustion engine on the basis of the concentration of oxygen contained in the exhaust.
Many systems have been proposed for detecting the A/F ratio of a combustion mixture supplied on a burner device such as an internal combustion engine on the basis of the concentration of oxygen in the exhaust gas. In a system described in Unexamined Published Japanese Patent Application No. 178354/1984, two elements are provided, each in the form of an oxygen-ion-conductive solid-electrolyte plate having a porous electrode formed on both sides, with the two plates disposed facing each other with a small gap therebetween. One of the elements is used as an oxygen pump cell for pumping oxygen out of the gap, while the other element is used as an oxygen-concentration-difference-actuated electrochemical sensor cell for generating a voltage in response to the difference between the concentration of oxygen in the ambient atmosphere and that in the gap. The system is capable of producing an accurate A/F ratio indicating signal at least in the fuel-lean region.
This A/F ratio detector, however, has the characteristic shown in FIG. 2. The system is primarily intended for generating an A/F ratio indicating signal in the fuel-lean region where residual oxygen is present in the exhaust gas, but even in the fuel-rich region where no residual oxygen is present, the system responds to CO, CO.sub.2, H.sub.2 O, etc., in the exhaust, producing a signal identical to that generated in the fuel-lean region. In other words, two values of A/F ratio are possible for the same value of the detection signal. In order to avoid this ambiguity, the system can be employed for A/F ratio control purposes only when it is definitely known whether the controlled burner device is operating in the fuel-lean or fuel-rich region. In the graph of FIG. 2, .lambda. denotes the excess air ratio, .lambda.=1 indicating an A/F ratio equal to the theoretical value.